1. Field of the Invention
The present invention relates to a film covered battery which contains a battery element sealed by a casing made of a film, and to a battery assembly which has a plurality of film covered batteries stacked in the thickness direction of battery elements.
2. Description of the Related Art
Conventionally, film covered batteries have employed a casing made of a thermally sealable film. A known such film covered battery has a battery element wrapped by a laminate film made up of a metal layer and a thermally sealable resin layer laminated thereon, and a positive and a negative lead terminal connected to the battery element and led out from the laminate film, with open edges of the laminate film being thermally fused to hermetically seal (hereinafter simply referred to as “seal” as the case may be) the battery element. This type of film covered battery advantageously facilitates a reduction in thickness, and therefore most of conventional film covered batteries are flat in shape.
As with batteries which employ casings made of other materials, a battery having a casing made of a film is also required to ensure reliability for the sealing in sealed regions to prevent outside air from introducing into the battery, and an electrolytic solution within the battery from leaking. Particularly, the reliability for sealing is important for a battery which includes a nonaqueous electrolytic solution (hereinafter also called the “nonaqueous electrolytic battery”). A defective sealing would cause deterioration of the electrolytic solution due to components of the outside air to significantly degrade the performance of the battery.
It has been often said that in a film covered battery, a portion of fused films from which lead terminals are led out is more susceptible to degraded sealability than the remaining portion, so that a leak path is readily formed for the outside air, and an electrolytic solution readily leaks from this portion unless appropriate measures are taken therefore. Once a leak path is formed for the outside air, the electrolytic solution will deteriorate due to components of the outside air, and water vapor included in the outside air will introduce into the battery, electrolyze on the surfaces of electrodes to generate a large amount of hydrogen, particularly in a nonaqueous battery, causing significant degradation in the performance of the battery. Also, the electrolytic solution, if leaking, would contaminate surroundings of the battery, and would stick to electric circuits around the battery to give rise to malfunctions of the electric circuits.
The degradation in sealability of lead-out paths for lead terminals may be faster or slower depending on how the battery is used and on the state within the battery. For example, if the internal pressure increases with the electrolytic solution remaining near the lead terminal lead-out paths, the pressure of the electrolytic solution is applied to the interface of the fused film, possibly contributing to promoted deterioration in sealability, and to advanced peeling of the fused film.
On the other hand, if the battery is applied with a voltage out of rating, the electrolytic solution will electrolyze to generate gas species which may cause an increased internal pressure of the battery. Further, if the battery is used at high temperatures out of a specified range, the electrolytic solution will also electrolyze to generate materials which can be sources of gas species.
Basically, it is ideal to use the battery within the specified range to avoid the generation of gas. However, it is difficult to completely eliminate any cause of generating a trace of gas within the battery depending on particular applications of the battery, even if the user intends to use the battery within the specified range, due to temporary control errors in a control circuit for the battery, instantaneous generation of a large current, lack of cooling for the battery, and the like, which would cause sudden or temporary generation of high temperatures.
To solve such troubles caused by the gas generated within the battery, film covered batteries have been proposed as illustrated below.
For example, JP-10-55792-A discloses a film covered battery which has a portion of a fused film that has a lower peel strength, such that when the internal pressure anomalously increases due to a gas generated within the battery, the gas is expelled from the portion having a lower peel strength.
JP-2000-133216-A discloses a battery which has an aluminum laminate film in a rectangular shape through draw forming to define a space for accommodating a battery element, thereby minimizing a surplus space.
JP-2000-100404-A discloses a battery pack which receives a battery sealed with a film in a battery room to constrain the battery fitted therein.
JP-6-111799-A discloses a battery which holds a space around electrodes, which make up a battery element, and wraps the battery element with an air-tight sheet made of synthetic resin for sealing the battery element.
As described above, while it is ideal to basically avoid the generation of gas within the battery, even a trace of gas is generated within the battery could accumulate within the battery over a long term of use. The gas thus accumulated within the battery will cause the internal pressure of the battery to increase, a film interface to be applied with a liquid pressure at lead terminal lead-out paths, thereby promoting deteriorated degradation and advanced peeling of a fused film in consequence. Particularly, if exposure of the battery element to the outside air can cause degradation in performance, the degradation in performance will arise, for example, with a nonaqueous battery. The degraded performance can result in a useless battery, and sudden exacerbation of charge/discharge characteristic depending on particular situations.
Reviewing the prior art techniques disclosed in the respective patent documents cited above from the foregoing point of view, the following problems can be pointed out.
In the battery disclosed in JP-10-55792-A, even if exposure of the battery element to the outside air causes degradation in performance, a portion of the fused film having a lower peel strength is operated as a safety valve which is opened to expose the battery element to the outside air. With this method, even a trace of gas generated during use of the battery will accumulate over a long term with an associated increase in the internal pressure. When the internal pressure exceeds a threshold, the portion having a lower peel strength automatically peel off even if the battery is used within a specified range. After the emission of the gas, outside air introduces from an opening formed due to the peeling. A nonaqueous electrolytic battery suffers from significant degradation in performance if outside air including moisture introduces into the battery, and falls into an out-of-service condition as the case may be. If the threshold is set at a low value, the battery will per se end up in an out-of-service condition in a short term.
In the structure of the battery disclosed in JP-6-111799-A, a casing is originally provided with an exhaust port, and a portion of the casing around the exhaust port is brought into close contact by means of oil to prevent an electrolytic solution from leaking from the exhaust port. However, while this structure can be basically applied to a battery such as lead storage battery which is not immediately affected by water vapor introduced thereinto from outside air to experience exacerbation of the battery characteristics, this structure cannot be used for a nonaqueous electrolytic battery because its battery characteristics are affected by water vapor of outside air introduced through the interface with such simple sealing provided by bringing the casing into close contact by means of oil.
On the other hand, it seems that the battery described in JP-2000-133216-A can withstand a relatively high internal pressure even if a gas is generated inside, if appropriate sealing is provided. However, in the battery disclosed in JP-2000-133126-A, the battery element room formed in the casing is matched in shape (size) with the battery element with the intention to minimize the spacing between the battery element and the casing for improving the volume efficiency, and moreover, the casing is thermally fused together near the battery element.
Describing with reference to FIGS. 1a and 1b, casing 111 is substantially identical in shape to battery element 113 before a gas is generated as illustrated in FIG. 1a. However, as a gas is generated, the casing is immediately deformed into a potbelly shape as illustrated in FIG. 1b because the gas cannot be saved up at the maintained atmospheric pressure. Since casing 111 is made of an aluminum laminate film, it hardly draw through elastic deformation, so that if even a trace of gas is continuously generated, the internal pressure will continuously increase.
The high internal pressure thus generated will finally provoke strong force F which peels off fused regions of casing 111 as illustrated in FIG. 1b, and the fused regions of casing 111 are eventually opened up to form a leak path. In other words, as this exemplary nonaqueous electrolytic battery is used over a long term, a trace of gas may be generated little by little, in which case the internal pressure readily increases to cause susceptibility to peeling of the fused regions of the casing and a leak due to the gas pressure, possibly causing the battery to fall into an out-of-service condition due to the introduction of outside air, as described above.
As described in JP-2000-100404, when a battery is kinematically constrained by a battery pack, a generated gas cannot be saved up at the maintained atmospheric pressure in the conventional structure. Therefore, if a trace of gas is continuously generated for a long term as mentioned above, the battery pack becomes more susceptible to peeling of a fused film and to a resulting leak due to the gas pressure.
Further, when a plurality of film covered batteries are combined into a battery assembly, the following problems may be envisaged.
In most cases, film covered batteries are flat in shape, so that when the film covered batteries are stacked in the thickness direction thereof to form a battery assembly, associated lead terminals are in close proximity to each other to facilitate electric connections. Also advantageously, a pair of holders may be used to simultaneously pressurize a plurality of unit batteries for maintaining the adherence for electrodes of battery elements. However, when flat film covered batteries are stacked and pressurized, the following problems can arise from a viewpoint of the sealability in regions of a film in which lead terminals are led out. Specifically, with conventional flat film covered batteries, stacked batteries sandwiched in between are difficult to volumetrically inflate, and therefore a generated gas will readily cause the internal pressure to increase. As such, the battery assembly is more susceptible to deterioration in sealability in the lead terminal lead-out paths due to the aforementioned mechanism unless appropriate measures are taken.
As described above, the film covered batteries have been highly required to improve the reliability for the sealing for the lead terminal lead-out paths to prevent the electrolytic solution from leaking. Also, when a plurality of film covered batteries are stacked to form a battery assembly, a disadvantageous influence has acted on the reliability for sealing the lead terminal lead-out paths.